Move peer db writes to eth2 libp2p (#2724)

## Issue Addressed
Part of a bigger effort to make the network globals read only. This moves all writes to the `PeerDB` to the `eth2_libp2p` crate. Limiting writes to the peer manager is a slightly more complicated issue for a next PR, to keep things reviewable.

## Proposed Changes
- Make the peers field in the globals a private field.
- Allow mutable access to the peers field to `eth2_libp2p` for now.
- Add a new network message to update the sync state.

Co-authored-by: Age Manning <Age@AgeManning.com>

beacon_node/network/src/sync/backfill_sync/mod.rs

@@ -213,14 +213,7 @@ impl<T: BeaconChainTypes> BackFillSync<T> {
         match self.state() {
             BackFillState::Syncing => {} // already syncing ignore.
             BackFillState::Paused => {
-                if self
-                    .network_globals
-                    .peers
-                    .read()
-                    .synced_peers()
-                    .next()
-                    .is_some()
-                {
+                if self.network_globals.peers().synced_peers().next().is_some() {
                     // If there are peers to resume with, begin the resume.
                     debug!(self.log, "Resuming backfill sync"; "start_epoch" => self.current_start, "awaiting_batches" => self.batches.len(), "processing_target" => self.processing_target);
                     self.set_state(BackFillState::Syncing);
@@ -906,8 +899,7 @@ impl<T: BeaconChainTypes> BackFillSync<T> {
         let new_peer = {
             let mut priorized_peers = self
                 .network_globals
-                .peers
-                .read()
+                .peers()
                 .synced_peers()
                 .map(|peer| {
                     (
@@ -1026,8 +1018,7 @@ impl<T: BeaconChainTypes> BackFillSync<T> {
         let mut rng = rand::thread_rng();
         let mut idle_peers = self
             .network_globals
-            .peers
-            .read()
+            .peers()
             .synced_peers()
             .filter(|peer_id| {
                 self.active_requests